Shot peening represents a known example of a surface modification process that is used for enhancing the fatigue strength of metallic materials such as the structural members used in aircraft and automobiles and the like. Shot peening is a method in which, by blasting countless particles having a particle size of approximately 0.8 mm (the shot material) together with a stream of compressed air onto the surface of a metallic material, the hardness of the metallic material surface is increased, and a layer having compressive residual stress is formed at a certain depth.
Particles composed of an iron-based material such as cast steel are cheap, and unlike sharp materials such as glass are unlikely to damage metallic material surfaces even when crushed, and they are therefore widely used as shot materials.
In terms of improving the fatigue strength of aluminum materials by shot peening, the process mentioned below has been disclosed (see Non Patent Citation 1).
Non Patent Citation 1: T. Dorr and four others, “Influence of Shot Penning on Fatigue Performance of High-Strength Aluminum and Magnesium Alloys”, The 7th International Conference on Shot Peening, 1999, Institute of Precision Mechanics, Warsaw, Poland. Internet <URL: http://www.shotpeening.org/ICSP/icsp-7-20.pdf>